Demons

Angels have intellect and free will, like man. Some angels chose, at the beginning, to rebel against God's will and became demons, or evil spirits.

Just as good spirits help us, evil spirits seek to harm us by tempting us to sin.

Any baptized and believing Christian has the power to resist temptation, whether it comes from "the world, the flesh, or the devil" - that is, (a) from the external world of fallen human society, (be) from the internal world of our own fallen, selfish instincts, of body or soul, or (c) from the supernatural world of evil spirits.

No faithful Christicn can be demon-possessed against his will, though many will be severely oppressed and all will be tempted. The Church, like Christ, has the power to free those who are "possessed" by exorcising the evil spirits. Christ promised this gift to his Church (Mk 16:17). In most times and places, demon possession and exorcism are rare. But they are real.

If the devil is not real, the Bible lies (for example 1 Pet 5:8), and Christ was a fool, for he certainly believed in demons and in Satan (see, for instance, Luke 10:18).

Catholic Christianity:A Complete Catechism of Catholic Beliefs based on the Catechism of the Catholic Church by Peter Kreeft